The invention relates to a hybrid encoder for video signals in which the picture elements of a video picture are arranged into equally large data blocks, each data block being compared with the corresponding data block of the previous video picture by means of a subtractor and the corresponding data block of the previous video picture being derived from a picture store, the difference block obtained by means of the subtractor traversing a first processing circuit which subjects the difference block to a matrix transformation and in which this transformation is inversed in a feedback path by a second processing circuit with the inverse matrix and the regained difference block is applied to a first input of an adder while data blocks from the picture store are applied to the second input of the adder and the output of the adder is connected to the data input of the picture store.
A hybrid encoder as featured above is known from a conference report of the COST group (draft version document Sim 85/78 Report COST 211-bis Simulation Subgroup Meeting, Stockholm, 10-11 December 1985). A circuit diagram of the known hybrid encoder is shown in FIG. 1. The main purpose of the encoder is to transcode the video data from a video data source with a possibly small loss of information into a signal having a possibly small bit rate. In this process two encoding principles--hence the name hybrid encoder--are used: The interframe principle in which the correlation between temporally successive video pictures (this designation is used for frames and fields in this respect) is utilized and the intraframe principle in which the correlation of the video data within a video picture is utilized.
Before the actual encoding process the data must be preprocessed. This operation is effected by a pre-processing unit PP in the hybrid encode of FIG. 1. The data are applied to the encoder in blocks. Such a video data block comprises the data of given pixels of a video picture which are considered as elements of a quadratic matrix (for the significance of the terms used in connection with matrices, compare Wigner, E. P.: Group Theory; Academic Press New York and London 1959, pages 1-30). For example, a data block may consist of the chrominance values of the first eight pixels of the first eight lines of a video picture. Each video picture is split up into equally large data blocks by the pre-processing unit PP. In this process each data block has a given identification. The data block herein described as an example may be symbolized and characterized, for example by b.sub.11. Data blocks of successive video pictures which have the same identification are designated as corresponding data blocks.
Such a designation should also refer to data blocks of successive video pictures whose information contents are identical as much as possible. Data blocks corresponding to one another in this respect play a role in hybrid encoders in which a so-called block matching process is performed. However, this hybrid encoder variant will not be further described.
When applying a data block, for example the data block b.sub.11 to an input of a subtractor SR the corresponding data block--designated by, for example b.sub.11 --of the previous video picture is simultaneously applied from a picture store BS to the other input of the subtractor SR. The subtractor SR derives the difference between the two blocks in the sense of the difference between two matrices (compare Wigner, page 7); this difference block is then subjected to further operations.
A first processing circuit F performs on each difference block a transformation in the sense of a matrix transformation (compare Wigner, page 9). If a is the symbol for the transformation matrix of the circuit F and d.sub.11 is the symbol for the matrix of the difference block--or more simply for the difference block--the block D.sub.11 =a.sup.-1 d.sub.11 a is present after the transformation at the output of the circuit F, in which a.sup.-1 is the symbol for the matrix which is inverse to a. The transformation by the circuit F approximately corresponds to the Fourier transformation (more precisely, a special two-dimensional Fourier transformation is concerned) in the acoustic signal transmission; the block D.sub.11 can generally be encoded with fewer binary digits than the block d.sub.11.
Subsequently the transformed signal transverses a quantizer Q which ensures another data reduction. In order that the overal signal can be transmitted with a constant bit rate to a receiver, a buffer store P is provided. A multiplexer MUX interlaces the useful signal read from the buffer store P with control information which in the relevant case serve inter alia for adjusting a quantizer at the receiver end (an adaptive quantizer is meant here). After the quantization the signal is also fed back via a feedback path to the input of the hybrid encoder. Initially the block D.sub.11 modified by the quantizer Q is regenerated by a regeneration unit (not shown) to such an extent that it corresponds to the original block D.sub.11, but for round-off errors. It is then transformed by a second processing circuit IF with the transformation matrix a.sup.-1 into the difference block d.sub.11 again (likewise disregarding round-off errors). Due to the connection of an output A of the picture store BS to an input of an adder AR, the adder AR adds to this block the prior data block b.sub.11 from which the difference block d.sub.11 was formed by the subtractor SR. Possible delays due to finite processing times are either compensated by delay members or phase differences of control clock pulses (both of which are not shown in FIG. 1).
As can be easily ascertained, the replacing data block b.sub.11 is present at the output of the adder AR (round-off errors are not taken into account) of the video picture supplied via the pre-processing unit PP. This data block is entered into the picture store BS via an input E and takes over the role of the prior data block b.sub.11 which is now erased.
In the known hybrid encoder the bit rate reduction is dependent on the construction of the quantizer Q. If the quantizing intervals are large, this also applies to the bit rate reduction, than the picture quality is reduced in a particularly disturbing manner by the high quantization errors. For example, a chess-board pattern may be visible on the display screen, which is the result of the split-up of a video picture in blocks, or permanently changing structures (artifacts) are produced and disappear again which may lead to a complete distortion of the displayed picture.
In order to reduce the quantization error at least for sensitive signal portions, for example adaptive quantizers are used, i.e quantizers whose characteristic is varied by the video signal. The design is elaborate inter alia because additional information about the state of the quantizer is to be transmitted to the receiver.